1. Field of the Invention
The present invention relates to an output-buffer switch for an asynchronous transfer mode (ATM) applicable to a digital communication network and capable of decomposing various kinds of data, e.g., audio data, video data and data for computer use, into data having a predetermined length, or cells as generally referred to, and performing switching operation on a cell basis. More particularly, the present invention relates to a control system for an output-buffer switch of the type accommodating ATM lines and switching them on the basis of a header included in a cell.
2. Description of the Related Art
Regarding a broadband ISDN (Integrated Services Digital Network), ATM is a promising transmission and switching system which implements multimedia communication combining video data, audio data, high-speed data, etc. ATM decomposes such data into cells having a predetermined bit size and performs switching and transfer on a cell basis.
In the ATM switching system, an ATM switch performs switching for routing each input ATM cell to a particular output port. In principle, the ATM switch analyzes a header added to a cell and then performs self-routing based on a hardware architecture. With the self-routing scheme based on hardware, it is likely that a plurality of cells concentrate on a single output port, resulting in contention. In light of this, it has been customary to provide the ATM switch with a buffer for dealing with contentions. ATM switches are generally classified into an output buffer switch, an input buffer switch, a cross-point buffer switch, and a shared buffer switch, depending on the location of the buffer. In the output buffer switch, buffers precede respective output ports and receive cells input via all input ports and then multiplexed; the cells are read out of each buffer at a speed matching output lines. The input buffer switch has buffers at the inlet of a switch matrix and outputs cells such that they do not conflict on an output highway. The crosspoint buffer switch has a buffer at each crosspoint of a switch matrix. Further, the shared buffer switch is so constructed as to accommodate all the input and output lines in a single cell buffer. Such four different switch configurations are outlined in FIG. 5 of the accompanying drawings.
In any one of the ATM switch arrangements stated above, the buffer size cannot be increased beyond a certain limit due to limitations regarding hardware design and production. It is a common practice to discard, depending on the concentration of traffic, cells unable to be stored in the buffer within a statistically allowable range, but at the expense of communication quality. In this respect, overflow control for reducing the cell loss probability is one of important techniques for ATM switches. The present invention relates to, among the various kinds of ATM switches, the output buffer switch and, more particularly, overflow control for this kind of switch. For details of an output buffer switch, a reference may be made to H. Suzuki et al "Output-Buffer Switch Architecture for Asynchronous Transfer Mode", International Journal of Digital and Analog Cabled System, Vol. 2, pp. 269-276, 1989.
Usually, the output-buffer switch for ATM is arranged such that when the output buffer is about to overflow, the input of cells to the entire input ports is restricted.
Even the output-buffer switch for ATM may be provided with buffers at the input port side in addition to buffers at the output port side. However, the problem with this kind of switch is that even when only one of the output buffers has overflown, all the input cells are restricted; that is, it lacks an implementation for identifying the overflown output buffer and restricting only the cells addressed thereto. This results in the head-of-line (HOL) effect at the input port side which is particular to the input buffer switch. The HOL effect refers to an occurrence that despite that the subsequent cells are not addressed to the overflown output buffer, they are stopped at the input buffer, degrading throughput to a critical degree.
Some users may desire low tariff rather than high speed transfer services, while some users may desire low speed transfer services which guarantee low cell loss probability and provides high quality communication services. To meet such demands, there is available a system capable of preventing cells for a call, to which a particular class is assigned, from being discarded by use of an exclusive buffer. However, this system is applicable only to a particular kind of cells.